1. Field of the invention
This invention relates to methods of preparing doped crystalline materials and more particularly to preparing sensitized lithium fluoride crystals suitable for use in thermoluminescent radiation detectors.
2. Brief description of the prior art
Thermoluminescent radiation detectors have long been used for dosimetry of ionizing radiation, particularly in personal radiation monitoring badges. A typical thermoluminescent material for use in such badges is a transparent crystalline material, e.g. lithium fluoride, having a low concentration of a dopant, e.g., calcium, magnesium, titanium, manganese, europium, copper, carbon, phosphorus or the like, atomically dispersed therein to form traps within the band gap between the highest filled band and the conduction band of the crystal. When such a doped crystal is irradiated with ionizing radiation, electrons and/or holes are formed within the crystal and migrate until they are trapped by atoms of the dopant. When the irradiated crystal is subsequently heated from room temperature to a temperature of a few hundred degrees Celsius, these trapped electrons and/or holes are thermally excited from the traps and recombine with emission of light. The total amount of light emitted is a measure of the electric charge stored in the traps and, accordingly, is a measure of the dose of radiation to which the sensitized crystal has been subjected.
Dosimeters for use as personal radiation exposure monitoring badges and the like are prepared using chips cut from a grown crystal or chips cut from a pressed slug of ground crystal or by mounting a thin layer of finely divided thermoluminescent crystals on a suitable substrate. These badges are worn for a predetermined period of time after which the thermoluminescence of the radiation sensitive layer is measured to determine the total radiation exposure accumulated over the measuring period.
Conventionally, as disclosed in Swinehart, U.S. Pat. No. 3,320,180, doped LiF for use in thermoluminescent radiation detectors is prepared by melting a mixture of LiF and the desired dopants in the selected proportions and growing a single crystal from the melt, e.g., by the Bridgeman method or the Czochralski method. The doped crystals so prepared are then crushed to an appropriate size and the grains so formed are either mounted on a generally planar surface or pressed to form a slug which is subsequently cut into chips to prepare a thermoluminescent dosimeter. This procedure, while it produces thermoluminescent radiation-sensitive elements of good quality, is time-consuming and requires handling of molten salts. Consequently, the conventional method is rather expensive.
Another method of preparing a thermoluminescent material is disclosed by Matsuzawa in U.S. Pat. No. 4,129,519. According to the process there disclosed, LiF powder is mixed with activator powders, e.g., magnesium fluoride, copper(II) chloride and the like, and heated in a crucible in an inert gas atmosphere at a temperature of 700.degree. C. to 1100.degree. C. for a period of 30 minutes to 3 hours. The material is then granulated, washed successively with acid and water and dried. The examples of the Matsuzawa patent teach heating the mixture of lithium fluoride, magnesium fluoride and a second activator to a temperature of about 1050.degree. C., well above the melting point of LiF (about 845.degree. C.), for a period of 30 minutes, cooling the mixture to room temperature, milling the solid material so prepared to a particle size of 90-200 mesh, washing with 1N hydrochloric acid and water, and drying. The thermoluminescent properties of the material so prepared are described, but the preparation of the thermoluminescent radiation sensitive elements is not described.
Holzapfel et al., U.S. Pat. No. 4,510,174, disclose the preparation of thermoluminescent radiation-sensitive elements in thin wafer form by hot pressing a previously sensitized powder of LiF onto the surface of a solid substrate. The sensitized LiF powder is pressed onto the surface of a substrate, e.g., a surface of aluminum, magnesium, aluminum oxide and the like, at a temperature and pressure sufficient to cause plastic flow of the powder and bonding to the substrate surface. Typically the hot pressing is conducted at a temperature of 400.degree. C. and a pressure of 75 megapascals (MPa) (10,875 pounds per square inch). The sensitized LiF powder used in this process is prepared by conventional means. Consequently, the hot-pressing step is an additional step added to the steps of forming a thin layer detector.
Accordingly, a need has continued to exist for a simpler and more economical method for preparing doped LiF crystals for use in thermoluminescent radiation detectors.